Attenuation of radio frequency (RF) waves is a common phenomenon inside a building. The attenuation is due to obstacles such as the walls, pillars, partitions, etc. in the building that obstruct the propagation of the RF waves. Consequently, the quality of RF signal coverage inside a building is poor as compared to an open area. Designs of systems for enhancing in-building RF signal coverage have traditionally been a challenge for RF design engineers.
It is impractical to design outdoor base transceiver stations (BTS's) that are able to also provide indoor or in-building RF signal coverage. Typically, in-building RF signal coverage has been provided using an indoor BTS. This indoor BTS is able to provide uniform radio signal coverage in different parts on different floors of a building. Another means of providing in-building RF signal coverage is by using one or more distributed antenna systems that include coaxial cables and fiber optic cables. Although the indoor BTS and the distributed antenna system both enhance RF signal coverage within a building, they are unfortunately inadequate and not cost effective for providing reasonably good RF signal coverage in enclosed environments, such as within lift shafts, lift cars in the lift shafts, underground mines, tunnels etc.
One solution for extending RF signal coverage inside for example a lift car is by providing a distributed antenna in the lift lobby of each and every floor of a building. Although such a solution works to some extent for providing RF signal coverage inside a lift car, the RF signal coverage is not seamless, especially when the lift car moves between floors in the lift shaft. Such a design also places a constraint that an antenna has to be mounted in each of the lift lobbies, resulting in a system that is relatively high in cost.
Another solution is disclosed in U.S. Pat. No. 5,603,080 wherein a leakage coaxial cable is run along the length of a tunnel. However, a break in the cable would render a section of the cable non-operational to result in severe radio fading near that section. If the leakage coaxial cable is long, amplifiers or repeaters are required along the length of the cable. These amplifiers or repeaters result in a higher cost of the solution.